Here “differential pair of transistors means a set of two parallel branches each comprising a transistor having an emitter, a base and a collector, the emitters being joined together and powered by a common current source such that the current from the source can only be shared between the two transistors. The bases of the transistors receive different electrical voltages and it is the difference between these voltages that adjusts the proportions of this sharing. The current source is further connected to a first power supply terminal. The collectors of the transistors are connected, directly or indirectly, to another supply terminal.
In all that follows the generic terminology transistor emitter, base and collector will be used (terms generally used for bipolar transistors), even if the transistors are field effect transistors (MOS transistors). In the latter case, the words emitter, base and collector should be taken to mean source, grid and drain respectively (terminology generally used for MOS transistors); the invention is applicable to MOS transistor circuits in the same way as to bipolar transistor circuits.
Typical applications in which a network of several differential pairs of transistors are met and to which the invention is applicable, are the following:                analog-to-digital conversion circuits with multiple comparators, in which each comparator comprises a differential pair receiving a voltage to be converted as an input on the one hand and a reference voltage on the other;        analog-to-digital conversion circuits with folding circuits, in which a folding stage comprises several folding cells each consisting of at least one differential pair, the current outputs of the cells being connected to one another for establishing a voltage or an analog output current that varies in a bell shape or sinusoidally according to the input voltage to be converted, the different cells each receiving the input voltage and a respective reference voltage.        
It is recalled that the base-emitter voltage of a bipolar transistor (at a temperature assumed to be constant) is proportional to the logarithm of the current that flows through the emitter; and the current that flows through the emitter for a given base-emitter voltage is proportional to the effective surface of the emitter; accordingly, two identical transistors of a pair are, at least in theory, traversed by the same current when their bases are brought to the same potential.
Analog conversion circuits are based on this property for establishing a precise conversion; the application of the same voltage to be converted and several different reference voltages to several differential pairs can be used to set the voltage to be converted very precisely in relation to the various reference voltages.
In precision analog circuits using several differential pairs, it is noticed, however, that operating inaccuracies (particularly inaccuracies of conversion) can result from the fact that the emitter-base voltages of the different transistors of these pairs are not exactly identical even when they are traversed by identical currents.
This is because the technologies are not perfect and two transistors manufactured simultaneously, having at least theoretically the same emitter surfaces, and even placed side by side in an integrated circuit and therefore having every chance of being identical, do not have strictly identical characteristics. Consequently, for the same base-emitter voltage of the two transistors in a differential pair, the emitters are not traversed by exactly the same current. Or reciprocally, a slight offset voltage (called the offset voltage of the differential pair) must be applied between the two bases (the emitters being assumed to be joined) for them to be effectively traversed by the same current.
This results in an inevitable dispersion of production. In addition, due to the very fact of this dispersion, the different differential pairs of an integrated circuit inevitably present different offset voltages from one another.
Within the set of differential pairs of the same multiple pair circuit can be observed a dispersion of offset voltages that responds to statistical laws, typically a Gaussian one whose standard deviation is inversely proportional to the square root of the effective surface of the transistor's emitter. In fact, it is found that the dispersion is greater for small transistors and smaller for large transistors.
It is therefore known that larger transistors should be used to improve the statistics for matching differential pairs and ending up with a greater accuracy of conversion. But then the capacitances are larger and the circuits are therefore slower, which is not desirable in applications such as fast analog-to-digital converters. For the latter, it would be better to have smaller transistors in the differential pairs.
It is therefore hard to reconcile both of these two characteristics of accuracy and speed which are nevertheless essential in circuits such as analog-to-digital converters.